The present invention relates generally to technology for detecting an analyte. In various embodiments, the invention relates to devices for measuring pH, the potential of hydrogen, which is a measure of the acidity or alkalinity of a solution. The pH of a solution is determined by the concentration of dissolved hydrogen ions (H+) (also referred to as hydronium ions, H3O+) within the solution. As the concentration of dissolved hydrogen ions within the solution increases, the solution becomes more acidic. Conversely, the solution becomes more basic as the concentration of dissolved hydrogen ions within the solution decreases. The concentration of dissolved hydrogen ions within a solution has traditionally been measured with a glass electrode connected to an electronic meter that displays the pH reading. Traditionally the terms “probe” and “electrode” have been used interchangeably to describe a functional grouping of component electrodes. As used herein, the term “electrode” is used to refer to a specific electrode in a probe, i.e., such as a “working electrode”, a “reference electrode”, or a “counter electrode”, and “probe” refers to a functional grouping of electrodes sufficient to generate a signal that can be processed to generate a reading indicative of the concentration of an analyte of interest in a solution.
The traditional glass pH probe has a working electrode (WE) that is an ion-selective electrode made of a fragile, doped glass membrane sensitive to hydrogen ions. The pH-responsive glass membrane is the primary analyte sensing element in this type of probe and so is referred to as the “working” electrode. Hydrogen ions within the sample solution bind to the outside of the glass membrane, thereby causing a change in potential on the interior surface of the membrane. This change in potential is measured against the constant potential of a conventional reference electrode (RE), such as an electrode based on silver/silver chloride. The difference in potential is then correlated to a pH value by plotting the difference on a calibration curve. The calibration curve is created through a tedious, multistep process whereby the user plots changes in potential for various known buffer standards. Traditional pH meters are based on this principle.
The response of traditional glass working electrodes (and probes and meters containing them) to pH is unstable, and glass probes periodically require careful calibration involving tedious, time-consuming processes, multiple reagents, and a well trained operator. The special properties and construction of the glass probes further require that the glass membrane be kept wet at all times. Thus, routine care of the glass probe requires regular performance of cumbersome and costly storage, rinsing, cleaning, and calibration protocols performed by a well trained operator to ensure proper maintenance and working performance.
In addition to tedious maintenance and storage requirements, traditional glass probes are fragile, thereby limiting the fields of application of the glass probe. In particular, the fragile nature of the glass probe makes it unsuitable for use in food and beverage applications, as well as use in unattended, harsh, or hazardous environments. Accordingly, there is a need in the art for pH probes and meters (as well as other analyte probes and meters) that address and overcome the limitations of traditional pH probes and meters employing the glass probe.
In response to the limitations described above for traditional glass probe pH measuring systems, voltammetric systems were proposed to offer a more robust system for the determination of pH. In a voltammetric system, an electrical potential is applied in a controlled manner, typically varied linearly with time, and the corresponding current flowing through a conductive material is monitored by means of, for example, a potentiostat (see, for example, Wang, “Analytical Electrochemistry,” 3rd ed, John Wiley & Sons, 2006). Initial proposals (see U.S. Pat. No. 5,223,117) were based on the concept of a WE composed of a conductive substrate with a redox active molecule attached to its surface. The hypothesis was that, provided an appropriate “analyte-sensitive”, redox active material (ASM) was used, the potential at which the maximum current flows in this system would be a function of the pH of the analyte solution. However, this initial proposal met with little enthusiasm, perhaps because it was demonstrated with an electrode that used gold as a substrate.
Significant advances were made in both theory and research laboratory practice of voltammetry-based analyte sensing systems when researchers discovered that carbon could replace gold as the conductive substrate and, moreover, that, regardless of the substrate, mixtures of redox active materials could be used in voltammetric systems (see PCT Pub. Nos. 2005/066618 and 2005/085825). One particularly intriguing proposal by these researchers was that a mixture of “analyte-sensitive” materials (ASMs) and “analyte-insensitive” materials (AIMs) could be attached to a conductive substrate and effectively convert it into both a WE (signal generated by the ASM) and a reference electrode (RE) (signal generated by the AIM). No significant advances, however, in either theory or practice were made for some time after these initial proposals and research (see, e.g., PCT Pub. Nos. 2007/034131 and 2008/154409).
The next significant advance in the field occurred when scientists discovered that, in practice, no redox active material is completely “analyte-insensitive” and that practical application of voltammetric technology should focus on WEs without AIMs. These scientists also discovered, however, that, regardless of whether a redox active material was characterized as an ASM or AIM, it could be made truly analyte-insensitive by sequestration in an ionic medium. This discovery led to the analyte-insensitive electrode or AIE, which could not only be used as a replacement of the conventional RE in traditional pH measuring systems but could also be used with WEs based on voltammetry. See PCT Pub. No. 2010/104962. Soon after these discoveries, pH meters suitable for use on the laboratory benchtop and for important research and development applications were created. See PCT Pub. Nos. 2010/111531 and 2010/118156.
However, despite these highly promising advances, in practice, the useful lifetime of, and suitable applications for, these probes were dependent, among other factors, on the means by which the redox active molecules were affixed to the substrate surface of the electrode. For example, the two main approaches for attaching the redox active molecule to the substrate of the electrode, non-covalent adsorption and direct chemical (covalent) linkage, both exhibit rapidly decaying signals, some losing much of their functionality in less than two days, and instability in certain analyte solutions or environmental conditions.
There is, therefore, a need in the art for methods and compositions for fixing redox active materials to the conductive substrate of an electrode for use in a voltammetry-based analyte-sensing system and for electrodes, probes, pH meters, and other analyte sensing devices based on voltammetric systems that provide longer useful lifetimes and can be used for a wider variety of applications. The present invention meets these needs.